#10365: Implement Semigroup as a superclass of Monoid Proposal (Phase 1) -------------------------------------+------------------------------------- Reporter: gidyn | Owner: quchen Type: feature request | Status: patch Priority: high | Milestone: 8.0.1 Component: libraries/base | Version: 7.10.1 Resolution: | Keywords: report-impact Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1284 Wiki Page: | -------------------------------------+------------------------------------- Comment (by ekmett): @rwbarton: The short version of it is this: We have `mconcat` for `Monoid` today. It actually sees a fair bit of use and I've seen folks implement it in the wild to do things like Kahan summation or better short circuiting than the base monoid can (it gets to assume an associativity.) `fold` gets to know the container, `mconcat` gets to know the Monoid. The moral equivalent in `Semigroup` requires some form of canonical non- empty type to fold over. The net effect of removing it is that the `semigroups` package would continue to exist just to supply this type, the class would lose its canonical structure to fold over, and `sconcat` becomes unimplementable. Why should moving `Semigroup` into `base` cost users functionality? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10365#comment:31 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler